The present invention relates to a process for the production of fluorocarbons.
Hydrofluorocarbons (HFCs), i.e. compounds containing substantially only carbon, hydrogen and fluorine and no chlorine, and in particular pentafluoropropanes, are increasingly being used to replace the environmentally disadvantageous chlorofluorocarbons (CFCs) in refrigeration systems as foam blowing agents and other applications. Furthermore, it is generally commercially desirable for commercially available HFCs to be as free of hydrochlorofluorocarbons (HCFCs), i.e. compounds containing substantially only carbon, hydrogen, fluorine and chlorine, and CFCs as possible. This preference has arisen, in large part, from a widespread concern that CFCs, and to a lesser extent HCFCs, are detrimental to the Earth""s ozone layer. As a result, there is a worldwide effort to use halocarbons which contain fewer chlorine atoms. In fact, some current regulations call for HFC products to contain not more than 0.5 weight percent total CFCs as an impurity, and these regulations may become more restrictive in the future. It is therefore important that certain commercial HFCs have a concentration of CFCs and HCFCS that is as low as possible.
In this regard, 1,1,1,3,3-pentafluoropropane (HFC-245fa) is considered to be a hydrofluorocarbon having zero ozone depletion potential, and is being considered as a replacement for chlorofluorocarbons in foams, refrigeration and other systems. The production of hydrofluorocarbons has been the subject of interest to provide environmentally desirable products for use as solvents, foam blowing agents, refrigerants, cleaning agents, aerosol propellants, heat transfer media, dielectrics, fire extinguishing compositions and power cycle working fluids.
It is known in the art to produce HFCs by reacting hydrogen fluoride with various hydrochlorocarbon (HCC) compounds. As is well known in the art, this type of reaction is often used in the manufacture of HFC-245fa, as is disclosed, for example, in U.S. Pat. No. 5,763,706xe2x80x94Tung et. al., which is assigned to the assignee of the present invention and which is incorporated herein by reference. It is common in processes involving this type of reaction to create a reaction product that contains un-reacted hydrogen fluoride, other un-reacted starting reagents, intermediate HCFC products, and the desired HFC. In such processes, a need exists to separate from the reaction product undesirable by products and un-reacted starting materials, particularly HF.
In the manufacture of certain HFCs, the reaction product is such that conventional distillation techniques can be used to separate the HF and the HFCs contained in the stream. However, certain other HFCs have a boiling point that is very close to HF and/or forms an azeotrope with HF, and the reaction product containing these HFCs can not generally be effectively and fully separated from the unwanted components using conventional distillation.
Caustic scrubbing techniques are known to be effective for separating HF from HFCs and HCFCs, even when the HFC and/or HCFC form azeotropes with HF. However, such caustic scrubbing techniques are disadvantageous because the HF which is separated from the product stream can not readily be recycled to the fluorination reaction. This lost HF tends to increase the cost of producing the desired HFC. Therefore, it is generally desirable to provide processes in which the amount of un-reacted HF recycled to the reaction step is relatively high.
A technique which has been used to separate azeotropic HFC from un-reacted HEF, while avoiding the need for caustic scrubbing, is known in the art as pressure swing distillation. See U.S. Pat. No. 5,918,481xe2x80x94Pham, et al., which is assigned to the assignee of the present invention and which is incorporated herein by reference. In the pressure swing distillation process, as with certain other processes that involve the separation of azeotropic HFC from un-reacted HF, a stream containing a large percentage of the un-reacted HF is produced and recycled to the fluorination reaction. In addition to the HF, however, this recycle stream also frequently includes one or more HCFCs. For many of such processes, the HCFCs that are produced are readily further fluorinated in the reaction step, and therefore including these materials in the recycle stream is generally acceptable.
Applicants have come to appreciate, however, that in other classes of such manufacturing techniques, the process produces a stream that includes one or more HCFCs that are not readily further fluorinated in the reaction step and which also form azeotropes with HF. Applicants have further recognized that this class of process, when operated according to the prior art, is characterized by an ever increasing build-up of the non-reactive HCFC in the system that needs to be purged from the system on a periodic basis. Such purging operations are disadvantageous for many reasons, including the fact that the HFC production process must normally be discontinued during the purging operation.
Applicants have come to appreciate that many of the processes currently used to produce HFC-245fa, including those which use pressure swing distillation to separate HFC-245fa from un-reacted HF, suffer from this problem of having relatively unreactive, azeotropic HCFCs in the reaction product. Applicants have discovered a process which overcomes the above-noted deficiencies in processes of this type, as explained in detail hereinafter.